Skip to main content

Whirling Watcher

Science Snack
Whirling Watcher
When you view short bursts of moving images, you see some interesting effects.
Whirling Watcher
When you view short bursts of moving images, you see some interesting effects.

A series of slits moving rapidly past your eye allows you to see images in short bursts. Such rapid but fragmented views of moving objects can make the objects appear to jerk along, change speed, or even move backward.

Tools and Materials
  • Access to a computer printer
  • Pattern disk (provided in the Assembly section below)
  • Scissors or utility knife
  • Cardboard or manila folder
  • Glue stick or tape
  • Pencil with eraser
  • Push pin
  • Large mirror (not shown)
  • Source of running water (not shown)
  • Black poster board to use as a background (not shown)
  • Black marker pen (not shown)
  • A partner
  1. Print the stroboscope pattern disk below (click to enlarge to full size and print from your web browser). Cut out the pattern and glue it to the cardboard or manila folder.
  2. Cut the cardboard to the same shape as the stroboscope, including the slits. You can cut with a good pair of scissors or use scissors in combination with a utility knife.
  3. Insert the push pin through the center of the printed stroboscope disk. Next, push the pin at an angle into the pencil eraser so that the disk stays in place. Make sure the disk is able to spin freely and you are able to look through the slits as you hold the pencil.
To Do and Notice

Stand facing a mirror, and hold the disk in front of you. Be sure the disk is mounted on the pencil so that the horses are facing the mirror. Close one eye, then spin the disk and watch its reflection in the mirror through one of the slits. Concentrate your attention on one of the horses, and you will see it gallop! Try spinning the disk faster, then slower, then in the opposite direction. Compare your results.

Try these other experiments:

Have a friend hold out a hand so you can see it through the spinning disk. Ask your friend to move the hand from side to side. Notice that the movement you see is jerky rather than smooth. Have your friend move his or her hand rapidly, then slowly. Notice that the amount of jerkiness changes as the speed of the hand movement changes.

Let water run slowly enough to produce a stream that breaks up into separate droplets as it falls. Place a black background behind the well-illuminated drops of water. Look through the spinning stroboscope and watch the water droplets fall in slow motion. Vary the stroboscope’s speed and see if you can make the water droplets stand still or even look like they are moving upward.

What’s Going On?

As the strobe disk rotates, a series of open slits moves rapidly past your eye. Each time a slit passes your eye, you see a glimpse of the scene on the far side of the disk. Each open-slit image lingers in your eye and brain long enough to merge with the next image. This phenomenon, called persistence of vision, can combine the glimpses in such a way that your brain perceives continuous motion.

If an object in the scene moves, your eye and brain can draw incorrect conclusions about that object’s motion. When you look at the stream of water, for example, one slit allows you to view a droplet in a particular position. Depending on how fast your strobe is turning, the next slit might let you see a different droplet just slightly below the position of the one previously viewed. Your eye-brain system interprets the combined views as the slow motion of a single droplet. If the second view catches the droplet in a position just above that of the previous view, the droplet will seem to rise.

Going Further

Place a bicycle upside down and spin a wheel. Look at the spinning spokes through the slits in the Whirling Watcher. You can see the spokes stop, or move slowly forward and backward, like the wheels on a moving stagecoach in an old Western movie. In modern Westerns, special wheels with unevenly spaced spokes are put on the stagecoaches to avoid the strange appearance of backward rotation when the moving wheels are filmed. A regular set of wheels with evenly spaced spokes is used for scenes in which the stagecoach is not moving.

You can exercise your creativity by making your own moving pictures. On the opposite side of the Whirling Watcher disk from the horses, in the space between each pair of slots, draw images, each of which is slightly different from its neighbors. (A running stick figure is an easy set of images to start with.) Look through one of the slots at a mirror, just as you did with the horses, and spin the disk.